There are millions (billions?) of photos you can use free of charge for your research or publication. See Copyright and Getting Started to learn those tricks.
Some image sources try to sell you free images. That's not illegal, just unconscionable. Before paying or asking for permission to use and image, find out if the charging institution has an exclusive to the image.
A good overall place to start is the Wikimedia page of online image collections. Necessarily always incomplete, it features proprietary collections, copyright-free and limited collections. A broad spectrum list.
A complementary list in many ways is Family Tree's 22 Online Historical Photos database.
Archives of the United States - Images - These "photos" turn out to be mostly photos of documents, but photography own by the NAUS will also be here.
Library of Congress Images - A rich collection of historic photographs, free for any kind of use.
George Eastman House Collection. - The museum collection of the father of American Photographic processes.
Smithsonian Photography Archive - Historic photos separate from the Library of Congress
New York Public Library photo collection - One of the great open-source collections.